Fluorescent chemosensors for metal ions

In this chapter, we will concentrate on the development of macrocycle-based fluorescent chemosensors for metal ions. Some examples from our laboratory will be discussed in detail. We have also inserted a few paragraphs dealing with the crowned spirobenzopyran ion-sensing systems since this new type of chemosensor has been developed only recently. 

Figure 1. Some fluorescent chemosensors for metal ions described by various authors.

Formica, M, Fusi, V., Giorgi, L., Micheloni, M (2012). New fluorescent chemosensors for metal ions in solution. Coord. Chem. Reo., 256, pp. 170-192, ISSN 0010-8545. 

CONTENTS Preface. George W. Gokel. Cryptophanes Receptors for Tetrahedral Molecules, Andre Collett, Jean-Pierre Dutasta and Benedict Lozach. Inclusion Polymerization in Steroidal Canal Complexes, Kiichi Takemoto, Mikiji Miyata. Functionalized Tetraazamacrocycles Ligands with Many Aspects, Thomas A. Kaden. Calixarenes as the Third Supramolecular Host, Seiji Shinkai, Kyushu University, Japan. Fluorescent Chemosensors for Metal and Non-Metal Ions in Aqueous Solutions Based on the Chief Paradigm, Anthony W. Czamik. Index. 

Bodenant B, Weil T, Businelli-Pourcel M et al (1999) Synthesis and solution structure analysis of a bispyrenyl bishydroxamate calix[4]arene-based receptor, a fluorescent chemosensor for Cu2+ and Ni2+ metal ions. J Org Chem 64 7034-7039... 

Despite the remarkable progress in the field of molecular recognition, it is still extremely difficult to predict a priori the structure of a selective, high-affinity ligand for a metal ion such as Hg ". The approach we intend to pursue is to take advantage of combinatorial chemistry methods—a collection of technical advances that allow one to seriously consider undertaking the synthesis of hundreds or even thousands of compounds simultaneously —to carry out the parallel synthesis of a set of fOO potentially selective high-affinity fluorescent chemosensors for mercury. 

The possibility of measurement of Mg2+ concentration in a matrix complicated by alkali and alkaline earth metal ions has been explored by titration of 68 (5 x 10-5 M) in 1 1 MeOH/H20 solution (pH 7.2) containing Na+ (5 x 10 3 M), K+ (1 X 10-3 M), Ba2+ (1 x 10 3 M), Sr2 (1 x 1(T3 M), and Ca2+ (1 x 10-4 M). The titration was monitored via fluorescence.125 The fluorescence intensity reached a maximum at 1 equiv of Mg2+, indicating that fluorescent intensity could be directly correlated to Mg2+ concentration. The lack of interference from the other metal ions present can be a result of their lower binding constants with 68 and lower quantum yields of the charged complexes of 68 with Na+, K+, Ca2+, Sr2+, and Ba2+ at this pH. Thus, ligand 68 possesses characteristics of an efficient fluorescent chemosensor for Mg2+ and may find use in determining Mg2+ in biological samples and, if immobilized on a solid support, may be incorporated into sensory devices for measurement of Mg2+ concentrations in aqueous solutions. 

Fluorescent chemosensors for the detection of heavy metal ions by a simple fluorescence tum-on (off-on) or quenching (on-ofi) are a fundamental tool for a rapid and nondestructive analysis of biochemical and environmental matrices (2007CSR993). A hot topic is the des p of selective sensors for Hg, one of the most hazardous pollutants of the environment. In this context, a new tren-based starburst molecule containing fluorinated 1,2,4-oxadiazoles as fluorophores has been synthesized and its sensing behavior towards several metal cations has been investigated by UV/Vis, fluorescence. 

Yoon and Guo describe boronic acid systems that can bind to nucleophilic species, 1,2-diols, and arylboronates that are converted into the corresponding phenols by treatment with hydrogen peroxide. Resulting in fluorescent chemosensors for carbohydrates, dopamine, fluoride, metal ions and hydrogen peroxide. 

An r/rr/mnv -squaraine-based chemosensor 23a absorbing at 635 nm (eM = 260,000 M 1cm 1) and emitting at 665 nm signals alkaline and earth-alkaline metal ions in millimolar concentrations in acetonitrile [81]. In presence of Na+ ions, the fluorescence signal weakly increases while it significantly decreases in presence of Ca2+, Mg2+, and Ba2+ and does not change substantially upon addition of K+ ions. The same squaraine 23a and the azacrown-squaraine 23b [82] were used for Na+ and K+ sensing in a plasticized PVC matrix [83]. The squaraine derivatives exhibited fluorescence emission based optical responses to... 

Fan L-J, Zhanga Y, Murphy CB, Angell SE, Parker MFL, Flynn BR, Jones WE Jr (2009) Fluorescent conjugated polymer molecular wire chemosensors for transition metal ion recognition and signaling. Coord Chem Rev 253 401 122... 

In order to quantify the transition metal ion concentration, Jones et al. [107] developed a highly sensitive fluorescent chemosensor in the form of dialkoxy-phenyleneethynylene-thiophene copolymers 68/69. The PAEs were functionalized on the thiophene unit with terpyridine (68), and included 2,2 -bipyridine (69) as a Lewis acid receptor. The terpyridine polymers [108] were found to respond quantitatively to transition metal ions at concentrations as low as 4x10 M (NP, Hg, Cr ", and Co " ). The additionally used bpy-PAE demonstrates that variation in the chelation at the receptor site is an important variable in tuning selectivity. The observed dynamic quenching mechanism, combined with the solubility of this material, provides the opportunity to extend these initial investigations to thin solid films for use in real-time monitoring applications. 

The detection of contamination with mercury is a very serious issue due to the extreme toxicity and ease of reaction of this heavy metal. Mercury is commonly used in many everyday applications, such as thermometry, barometry or scientific apparatus. Therefore, it is important to note the development of a fluorescent IIP membrane for detection of mercury [63], This membrane was based on the combination of two fluorescent functional monomers, namely 4-vinyl pyridine and 9-vinylcarbazole. Binding of Hg2+ was proposed to be based on complexation of the metal ion with lone pairs of both pyridine and carbazole. The IIP membrane was capable of recognizing Hg2+in the linear range of concentrations from 5 x 10 7 to 1 x 10 4 M. Selectivity of the chemosensor was studied for a wide group of possible competitive ions and the chemosensor responded to all of them in the presence of a fixed amount of Hg2+ at 1 x 10 5 M. Nevertheless, the highest influence on recognition properties was observed in the presence of Cu2+ (4.87%) and Pb2+ (4.57%), which are still acceptable values for the successful detection of Hg2+. This progress stimulates further development of the MIP detection of ions. 

PEBBLEs are water-soluble nanoparticles based on biologically inert matrices of cross-linked polymers, typically poly(acrylamide), poly(decylmethacrylate), silica, or organically modified silicates (ORMOSILs), which encapsulate a fluorescent chemo-sensor and, often, a reference dye. These matrices have been used to make sensors for pH, metal ions, as well as for some nonionic species. The small size of the PEBBLE sensors (from 20 to 600 nm) enables their noninvasive insertion into a living cell, minimizing physical interference. The semipermeable and transparent nature of the matrix allows the analyte to interact with the indicator dye that reports the interaction via a change in the emitted fluorescence. Moreover, when compared to naked chemosensors, nanoparticles can protect the indicator from chemical interferences and minimize its toxicity. Another important feature of PEBBLEs, particularly valuable in intracellular sensing applications, is that the polymer matrix creates a separate... 

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